Various forms of communications can be performed in packet-based networks, such as electronic mail, web browsing, file transfer, and so forth. With the increased capacity and reliability of packet-based networks, voice communications (along with other forms of real-time, interactive communications) have also become feasible. In such communications, voice and other real-time data are carried in packets that are sent across the network.
Various standards have been proposed for voice and multimedia communications over packet-based networks. One such standard is the H.323 Recommendation from the International Telecommunication Union (ITU). Another standard for voice and multimedia communications is the Session Initiation Protocol (SIP), as developed by the Internet Engineering Task Force (IETF). Generally, H.323, SIP, and other control protocols are used for negotiating session information to coordinate the establishment of a call session. Once negotiation setup has been completed, packetized media (including voice or other forms of real-time data) can flow between endpoints. A media transport protocol, such as the Real-Time Protocol (RTP), is used for conveying packetized media between the endpoints.
Various issues are associated with communications over packet-based networks. One is the dwindling supply of network addresses, such as Internet Protocol (IP) addresses. To address this problem, network address translation (NAT) is provided to enable address translations between public and private networks. By reusing a pool of private addresses in different private networks, the virtual supply of network addresses is extended. Another concern of packet-based communications is security. Once a network address of a specific node is known, this network address can be used as routing information to gain illegal access to the node and all of its resources. Network address translation can be used to hide network addresses of nodes to protect such nodes.
To communicate over a public network (e.g., the Internet), an endpoint typically goes through an intermediary system, such as a service provider. The service provider may offer various services, such as electronic mail, web browsing, voice communications, and other services. Such services can be offered by one or more nodes associated with the service provider. However, in communications between endpoints that involve a service provider or other intermediary entity, conventional address translation and security mechanisms may not provide the desired functions or features.